ALLEGRO DESIGN ENTRY HDL SI XL Cadence Allegro Design Entry HDL SI XL allows electrical engineers to develop and embed optimum constraints in printed circuit board (PCB) design intent. Tightly coupled with Allegro Design Entry HDL XL, it allows designers to leverage proven simulation technology to develop and specify constraints on nets as they are created. With Allegro Design Entry HDL SI, engineers can shorten design cycle time by enabling a constraint-driven PCB design process from the outset of a design.